Standardization of Gamma Analysis for the Evaluation of Pre-Treatment Quality Assurance of RapidArc Plans Using 2D Array Combined with Octavius Phantom

Purpose:To evaluate and standardize gamma analysis for different RapidArc plans.

Methods:45 different RapidArc plans were selected for the analysis study. Verification plan was created for each treatment plans in Eclipse 10.0 TPS using the AAA algorithm. Gamma analysis Pre-treatment quality assurance was done using 2D array combined with Octavius phantom. Analysis was done separately for head and neck, thorax and abdomen case using different dose difference (DD) and distance to agreement (DTA) passing criteria.

Results:The standard gamma passing criteria (3mm, 3%) shows a good pass percentage for all the sites with an average passing percentage of 95.49 (SD ± 3.75), 97.82 (SD ± 1.95) and 97.89 (SD ± 2.43) with head and neck, thorax and abdomen cases respectively. But when comparing head and neck and thorax cases, there is a significant difference between 1% DD and 3% DD for thorax cases. Also there is a significant difference between 1% DD and 3% DD with thoracic cases when compared to head and neck and abdomen cases. P-Value of 0.009 (for 1mm, 1% gamma), 0.003 (for 2mm, 1% gamma), 0.010 (for 3mm, 1% gamma) was observed when analysis was done comparing head and neck with thorax cases. Also significant changes were noted when comparing thorax cases with abdomen with P-Value of 0.001 (for 1mm, 1% gamma), 0.003 (for 2mm, 1% gamma), and 0.047 (for 3mm, 1% gamma). Same conditions were observed when comparison done for thorax and abdomen cases, with a significance difference with thorax cases.

Conclusion:It is recommended to standardize the gamma passing criteria for specific site rather than going for the standard (3mm DTA, 3% DD) criteria. Also it is suggested to choose lower DD passing criteria to know the mismatch areas in the fluence when ever analysis done with thoracic cases.